As winter approaches, shorter days spark a cascade of changes within the brains of animals. As a result, groundhogs retreat to their burrows for hibernation, weasels’ coats turn from brown to white, and birds cease their singing. And scientists are finding that seasonal behavioral changes similarly happen for some humans. Kelly Rohan digs into what we know about seasonal affective disorder. Lesson by Kelly Rohan, directed by Avi Ofer.